1. Field of the Invention
This invention relates to quick acting panel fastener assemblies in general and to a fastener assembly with improved retaining and locking means for stress access panels in aircraft and the like.
2. Description of the Prior Art
As is well known to those skilled in the art, the external surface or skin of aircraft is designed to provide a significant amount of structural strength. It is common for numerous access panels to be strategically located on aircraft through which access may be had to electronics, hydraulics, controls, and other equipment or structure within the aircraft. In order to maintain the structural efficiency of the aircraft skin, these panels must also be designed to carry their share of the load. Therefore, when a panel is removed the sub-structure is stressed due to the lack of support provided by the panel and as a result the sub-structure yields or deflects to a limited degree. The movement of the under structure occurs gradually as the fasteners are removed so that the last of the fasteners of the panel to be removed are the most affected by residual shear, making their removal the most difficult. In replacing a panel, the fasteners must have sufficient strength to pull the warped or deflected panel and the sub-structure tightly together and must have a large tolerance to enable them to pull the misaligned holes into place.
In view of the load carrying requirement of the access panels and the fasteners, it is extremely important that the fasteners are not lost during the removal of the panel from the aircraft. Loss of a sleeve bolt in many cases would require that the aircraft be grounded until a replacement could be located. Accordingly, it is extremely important that the sleeve bolts be secured or retained in the access panel when it is removed from the aircraft sub-structure.
Particular aircraft access panels range widely in size. The number of fasteners required to secure a particular panel to its mating sub-structure depends not only on the size of the panel but the location of the panel and the amount of stress which it was designed to carry. Accordingly, it is not uncommon for such panels to require a multiplicity of fasteners. In reinstalling a previously removed panel to its mating sub-structure it is necessary that the sleeve bolts, which are connected to the panel, be secured in their withdrawn position to aid in the rapid reinstallation of the panel.
Quick acting fasteners of the type described in U.S. Pat. No. 2,991,816 have been widely accepted for aerospace use as an effective fastener for joining a panel to a sub-structure. Such fasteners include a grommet assembly which is attached to the panel and through which a sleeve bolt passes to engage a receptacle assembly that is attached to the aircraft sub-structure. The grommet assembly retains the sleeve bolt in the panel when the panel is disengaged from the sub-structure and maintains it in a fully withdrawn position while the panel is disengaged to aid in rapid replacement of the panel.
Panel fasteners of the type described are relatively expensive, and since they form a portion of the aircraft structure, they must be highly reliable and remain locked once the threads of the sleeve bolt are fully engaged. To insure such a lock, the sleeve bolt is typically squeezed or deformed in order to provide local deformation of the internal threads therein to effect a high frictional force between the mating threads of the sleeve bolt and the receptacle assembly stud, as illustrated in U.S. Pat. No. 2,991,816.
Depending on the degree of deformation of the sleeve bolt, the mating threads of the sleeve bolt and stud gradually wear sufficiently to reduce the amount of friction therebetween, so that the locking feature is eventually reduced, and is sometimes completely lost. When this occurs, the relatively expensive sleeve bolt and receptacle assembly must be replaced in order to insure that a prevailing torque lock exists between the threaded members.
The requirement that the sleeve bolt must be retained in the panel structure when not threadably engaged on the receptacle assembly, is accomplished by means of a resilient spring member which engages a groove adjacent an enlarged flange on one end of the sleeve bolt. This resilient spring, as shown in U.S. Pat. No. 2,991,816, expands over the progressively increasing diameters of the sleeve bolt when it is advanced through a hole in the panel to engage the threaded receptacle assembly. The spring and sleeve bolt described in U.S. Pat. No. 2,991,816 are relatively expensive to manufacture and have not been entirely satisfactory in reliably and consistently retaining the sleeve bolt in the panel, when the panel is disengaged from the aircraft sub-structure.
U.S. Pat. Nos. 3,180,388, 3,221,589, 3,232,088 and 3,397,727 show split star-shaped retaining rings, as does U.S. Pat. No. 2,991,816. Such retaining rings are expensive to manufacture and are not always as effective as designed in retaining sleeve bolts to aircraft access panels.